Red Hat and Novell, the two top Linux sellers, have only just begun building Xen virtualization software into their products. But they're already planning to add a higher-level option. Xen is a 'hypervisor' that lets a single computer run several operating systems simultaneously, using an idea called 'virtualization'. This enables companies to use a single server more efficiently - something that could save them money. Now 'containers', a higher-level virtualization approach that makes a single operating system look like many, is also getting traction.